Similarly, Simigdala et al. [78] investigated the effect of abiraterone on ER signaling in a panel of ER+ breast cancer cell lines (MCF7, HCC1428, and SUM44) sensitive or resistant to long-term estrogen deprivation, modeling relapse on an aromatase inhibitor and containing different natural estrogen receptor 1 (ESR1) mutations. This evidence concerns the gene ESR1 and breast cancer.